X-ray fluorescence (XRF) measurement, and specifically X-ray microfluorescence (i.e., X-ray fluorescence using narrow, focused excitation beams), is gaining increasing attention as a method for testing semiconductor wafers. XRF itself is a well-known technique for determining the elemental composition of a sample. XRF analyzers generally include an X-ray source, which irradiates the sample, and an X-ray detector, for detecting the X-ray fluorescence emitted by the sample in response to the irradiation. Each element in the sample emits X-ray fluorescence in energy bands that are characteristic of the element. The detected X-ray fluorescence is analyzed to find the energies or, equivalently, the wavelengths of the detected photons, and the qualitative and/or quantitative composition of the sample is determined based on this analysis.
U.S. Pat. No. 6,108,398, for example, whose disclosure is incorporated herein by reference, describes an XRF analyzer and a method for analyzing a sample. The analyzer includes an X-ray beam source, which irradiates a small spot (on the order of 50 μm in diameter) on the sample via a monolithic polycapillary optic. The irradiation causes the sample to emit fluorescent X-ray photons. An array of semiconductor detectors is arranged around the spot so as to capture the fluorescent X-ray photons. The analyzer produces electrical pulses suitable for analysis of the sample.
The use of X-ray microfluorescence for testing semiconductor wafers is described in U.S. Pat. No. 6,351,516, whose disclosure is incorporated herein by reference. This patent describes a non-destructive method for testing the deposition and/or the removal of a material within a recess on the surface of a sample. An excitation beam is directed onto a region of the sample in a vicinity of the recess, and an intensity of X-ray fluorescence emitted from the region is measured. A quantity of the material that is deposited within the recess is determined responsively to the measured intensity.
Another application of X-ray microfluorescence is described by Lankosz et al., in a paper entitled “Research in Quantitative X-ray Fluorescence Microanalysis of Patterned Thin Films,” Advances in X-ray Analysis 43 (1999), pages 497-503, which is incorporated herein by reference. The authors describe a method for X-ray fluorescence microanalysis using a collimated micro-beam. The method is applied for testing the thickness and uniformity of thin films prepared by ion sputtering techniques.
As yet another example, U.S. Pat. No. 7,245,695, whose disclosure is incorporated herein by reference, describes a method for testing a material applied to a surface of a sample. An excitation beam is directed onto a region of the sample, and the intensity of X-ray fluorescence emitted from the region is measured. The beam may be scanned over a feature on the surface in order to produce a measured XRF profile of the scanned feature. The distribution of the material within the region is estimated based on the measured intensity of the X-ray fluorescence and on the known intensity cross-section of the excitation beam. The method is said to achieve a spatial resolution that is finer than the beam-width of the excitation beam.